The present invention relates to a method of heating, melting and cooling a substance held in a levitating state by a magnetic force produced by a gradient magnetic field, and also relates to a levitation-melting apparatus as well as a spherical substance manufactured threreby.
A demand for provision of a material, which is extremely purified or formed to an ideal shape with extremely smooth surface becomes stronger and stronger in response to progress of technologies such as electronics and optics. A conventional method for melting material uses a container such as a crucible or mold in general, but the melted material is likely to be contaminated with a substance dissolved from the container. In this regard, a melting method without use of a container has been developed in order to avoid contamination of a melted substance with the container.
A melting method for synthesis of a substance at a high temperature without use of a container is performed in a gravitational levitating state generated by an electrostatic force, an electromagnetic force, a force of gas stream or a force of acoustic waves or in a microgravity field generated in an outer space or by free fall using an airplanes or a drop tower. Such the method of melting a substance in microgravity levitating state was experienced in the outer space in 1992 using an acoustic levitating furnace, and its usefulness was recognized. However, it is necessary to establish a method which can realize a levitating state on the ground instead of the outer space, in order to make the microgravity levitating method applicable to industries with economical efficiency.
In an electrostatic method, a substance is electrostatically charged and held in a levitating state by attractive and repulsive forces produced by an electric field. Formation of a stable point in the electric field for levitating a substance at its center is necessary for realization of a stable levitating state. However, it is difficult to generate such an electric field having a stable point, and retention of an electrostatically charged state is practically impossible at a high-temperature atmosphere necessary for holding a substance in a molten state.
In an electromagnetic induction method, an induced current is generated in a substance by fluctuation of a magnetic field, so as to hold the substance in a levitating state by a repulsive force derived from the induced current. Due to a producing mechanism of the repulsive force, the substance shall be electrically conductive, but a levitating force does not effect on an insulator such as glass.
In a method using a gas stream, a substance is held in a levitating state without any restrictions on electroconductivity or chemical composition, and levitation of a massive substance is also possible by increase of a gas stream. However, increase of the gas stream makes it difficult to heat and melt the substance. The gas stream also causes big fluctuation of a melt, which is inappropriate for production of a spherical product or a product having a smooth surface.
By an acoustic method, a substance is held in a levitating state regardless of electroconductivity. But, a levitating force produced by an acoustic field is extremely small compared with an electromagnetic force or a force of a gas stream, so puts restrictions on a size or mass of a substance to which the method is applicable.
Fluctuation of a melt derived from high-frequency vibration in the method using electromagnetic induction or acoustic waves causes unfavorable material flow in a melt, resulting in mal-shape of a product.
In the methods using any of an electrostatic force, an electromagnetic induction force, a force of a gas stream and a force of acoustic waves, a substance is held in a levitating state by application of an upward force, so that atoms or molecules of the substance are in a state charged with a gravity of 1G. Consequently, vortex in a melt is unavoidable, when the substance is partially expanded or shrunk due to thermal distribution which occurs during a heating step. Ununiform distribution of components. is also unavoidable due to difference in specific gravity originated in chemical composition. Even if a levitating state free from fluctuation would be realized in such the method, an obtained spherical product is of a deformed shape due to affection of gravity applied on a surface of a molten substance.
A method using free fall realizes an extremely tiny gravity, since each gravity affected on atoms or molecules is offset together without substantial influences in response to a kind or size of a substance. There is neither vortex nor ununiform distribution caused by difference in specific gravity. However, a time period of free fall capable of keeping such a microgravity field is very short, if such the method is performed on the ground. For instance several seconds in a case using a drop tower, 20 seconds or so in parabolic flight of an airplane, and only a few minutes even in trajectory flight of a rocket. That is, it is actually impossible to complete all processes from melting to cooling steps necessary for production of a spherical product. Of course, a levitating state can be maintained over an infinitely long time in the outer space, but incommensurably huge cost is required.
As aforementioned, a method of melting and cooling a substance in a levitating state on the ground is not practical due to unresolved problems on chemical composition, electroconductivity or mass of a substance in :addition to a holding time period and stability of a microgavity field. In this regard, a new process, which maintains a microgravity field for a long time on the ground enough to melt and solidify a substance in a stable levitating state, has been demanded for production of a product having an ideal spherical shape with an extremely smooth surface.
The present invention aims at provision of such a process capable of production of a product having an ideal spherical shape with an extremely smooth surface. An object of the present invention is to realize a microgravity field suitable for heating, melting and cooling a substance without any time constraints by using a magnetic force produced by a gradient magnetic field.
According to the present invention, a substance is located in a gradient magnetic field, held in a levitating state by a magnetic force produced by the gradient magnetic field, heated and melted as such in the levitating state by ray or heat irradiation or by a resistance heater, and then cooled as such by stopping the ray or heat irradiation or the resistance heating.
A levitation-melting apparatus therefor comprises means for generation of a gradient magnetic field, means for holding a substance at a stable point of the generated magnetic field, and means for heating the substance with ray or heat irradiation or by a resistance heater. A product obtained by this method is of a spherical shape superior of smoothness due to a surface tension generated in a molten state.